1. Field of the Invention
This invention is related to high-speed signaling, and in particular, improved signal integrity and impedance termination in high-speed (e.g., gigahertz) differential signaling systems.
2. Description of the Related Art
As processors and memory technologies have become more powerful, demand for higher input/output (I/O) bandwidth has also increased for achieving optimal system performance. Various high-speed I/O technologies are being employed, for example, low voltage differential signaling, small computer system interface (“LVDS SCSI”) and the recent INFINIBAND™ (“IB”) architectural specification. The IB architecture addresses industry trends toward more modular and distributed architectures. IB encompasses I/O, such as server-to-storage communication and interprocessor communication between servers, and it uses high-speed, bi-directional serial links between each host channel adapter (HCA) and target channel adapter (TCA).
High speed signaling, however, tends to generate noise. For example, cables used in high-speed signaling systems, typically, the longest parts of the system, can act as efficient antennas that pick up and/or radiate noise. This may cause certain products to fail radiofrequency emission control specifications established by the Federal Communications Commission (FCC) or other regulating agencies in various countries. These specifications address electromagnetic compatibility (EMC) and/or electromagnetic interference (EMI). EMC is the ability of an electronic system, such as a computer, to function properly in its intended electromagnetic environment, without being a source of pollution. Designing computer equipment to exhibit effective EMC and minimize EMI is therefore important.
There are several known ways to minimize cable noise. Shielded differential cables, for example, can be used to improve EMC. Also, as is well known to those skilled in the art, perfect impedance matching of cables and system printed circuit boards (PCBs) is desirable to minimize signal reflection, potentially improving signal integrity and reducing the propensity for noise generation. But, perfect impedance matching can be difficult to achieve in practice, and may be cost prohibitive for most products. Also, as the speed of the transmission increases, any imbalance in the same differential signal pair caused by imperfect driver or cable skew can degrade signal quality and increase radiation. To maintain signal integrity and meet EMI requirements, this skew within differential signals driven by the same driver must be tightly controlled. The present invention addresses these and other problems associated with high-speed signaling systems.